This invention relates to a method and apparatus for sampling data pulses, such as at the output of a charge-coupled device (CCD), and more particularly to improving the signal-to noise ratio in the sampled data.
The development in recent years of charge-coupled semiconductor devices as a result of the MOS-FET technology has led to making analog data registers practical. An article by Gilbert F. Amilio entitled "Charge-Coupled Devices," pages 23-31 in Scientific American, Volume 230, No. 2, February 1974 explains the structure and analyzes operation of such a charge-coupled device (CCD). Another teaching of such devices can be found in U.S. Pat. No. 3,814,955 issued to Yokichi Itoh et al in 1974. Briefly, a CCD consists of a silicon dioxide layer deposited on a silicon substrate material. A plurality of metal transfer electrodes are deposited on the silicon dioxide layer. In N-channel devices, an N-material input diode is formed in the P-type silicon substrate to provide a diode charge-coupling into the device, and an N-material output diode is formed in the P-type silicon substrate to provide a diode charge-coupling out of the device, or a floating gate can be used to sense the charge. By pulsing the transfer electrodes in sequence, it is possible to move potential charges of minority carriers along the surface, or in a buried channel of the silicon substrate. Packets of electrons may be transferred into and out of potential wells of these charge-coupled devices virtually intact, and from potential well to potential well and out through the output diode.
Charge-coupled devices have been used not only in implementing radar data processing components, but also more recently in image sensing elements in a matrix for a full frame of image picture elements (pixels). A major advantage of charge-coupled devices is that once a frame has been stored as charge packets proportional to light intensity of the pixels, they may be read out serially from one CCD by shifting the last row out serially, and shifting the rows down in parallel, or simply queuing all of the rows for full serial-shift readout. In either case, a CCD is read out in response to a clock pulse from a stable source used for synchronization in a CCD data processing system. The problem has been achieving a sufficiently high signal-to-noise ratio to make CCD imaging effective for very faint image sources, such as in sensing stellar visual images. The output of CCD systems is conventionally obtained by a sampling technique using a double sampling technique.